GCE (A level) Physics E08 Resistance. Part 2 of 2

Part 2 of 2. Examines the resistance of ohmic resistors and the non-ohmic filamement bulb. Describes how current-voltage graphs are used to find resistance.

Category:

Education

Tags:

• GCE
• physics
• A-level
• AS
• A2
• electricity
• resistance
• ohm
• IV
• graph
• graphs

[xTruncz]

If the resistance of the filament lamp increases as the temperature increases then is that the same as saying it's Posotively Cooeficiant? And also isn't a Thermistor negatively cooeficant?

I have my first Physics exam of AS in like 8 day's, and I was really good at the subject it to start with, (my particle physics is at grade A but my electricty is much further behind at about grade D to U), but I've had to put Physics of for a long time now due to Chemistry! I need alot of help man :/

[Steve4Physics]

@xTruncz Hi. Yes, the filament has a positive temperature coefficient. Semiconductors are negative temperature coefficient, but thermistors can be made with both positive and negative temperature coefficients. I've sent you a separate email about this. Good luck

[Steve4Physics]

To songyosk.

The value of resistance (R) is the voltage (V) across a component divided by the current (I) through it. R=V/I, that’s how resistance is defined.

If you mark a point, P, on a curved V-I graph, the value of V/I at point P is the same as the gradient of the cord from the origin to P. The value of V/I at point P does not equal the gradient of the tangent at P.

[songyosk]

@Steve4Physics

I am confused because one of the video on Q-t graph, the gradient of the tangent was needed to find the current, I.

Because Q/t=I, so I thought this was the same case.

[Steve4Physics]

@songyosk

If you are told V=5volts and I=2amps, you know R = 2.5Ω - the shape of the V-I graph doesn’t make any difference. (But if it’s a straight line through the origin, then in this special case, R does equal the gradient.)

But if you are told the charge is 5C when t=2s, you do NOT tknow the current - the charge mught not have started flowing till t=1.99s for example - you need the gradient at t=2s.

[songyosk]

why is the gradient of the cord used instead of the tangent ?

[bobmarlyrob]

thanks alot this has helped me more